The synchronous machine is one of the most important types of electric machines. Traditionally, the synchronous machine is used for generating electrical energy, for example. The electric machine, in particular the synchronous machine, is increasingly gaining significance as a drive assembly in motor vehicles. Here, the synchronous machine can be used, for example, in purely electrically driven vehicles and also in hybrid vehicles. There are substantial differences between permanent magnet synchronous machines and separately excited synchronous machines. Permanent magnet synchronous machines have a rotor on which permanent magnets are arranged. In the case of a separately excited synchronous machine, use is made of a rotor with a winding through which a field current flows via slip rings, as a result of which a magnetic excitation flux or rotor flux is produced on the rotor side which interacts with a rotating field generated via energized stator windings.
Separately excited synchronous machines are used, for example, in hybrid vehicles, in particular in axle hybrid drive systems. Hybrid technology is substantially characterized by the fact that two drive assemblies, an internal combustion engine and an electric machine are arranged in the vehicle. In this case, the motor can be connected, for example, to an axle differential parallel to the axle via a two-stage spur gear. In particular in the case of motor vehicle drive assemblies, safety is a particular criterion which needs to be taken into consideration when selecting the type of electric machines. Ideally, it should be possible to disconnect the motor at any point in time. One advantage of the separately excited synchronous machine is the fact that the rotor-side field current can be disconnected, whereas, in the case of permanent magnet synchronous machines, the rotor-side magnetic excitation flux cannot be disconnected during normal operation. An important aspect in the case of electric machines in motor vehicles concerns the losses occurring during operation since said losses need to be supplied either completely from a rechargeable battery or via a generator coupled to an internal combustion assembly. Together with the converter losses, these are referred to as driving cycle losses, for example. The losses characteristic is a particular criterion in the design of the electric machine, in particular the separately excited synchronous machine, and the associated regulation with the converter.